Purpose: Weight gain in adulthood is a risk factor for breast cancer; however, the impact on age of onset is unknown. The objective of this study was to investigate whether weight gain from early- to mid-adulthood influenced the timing of breast cancer onset. Methods: Increase in body mass index (BMI) from lowest adult BMI to BMI at diagnosis and age at which these events occurred were calculated from breast cancer survivors enrolled in a weight loss trial (n = 660). Quartiles (Q) of the average increase in BMI were determined and associations between weight gain and age at disease onset were analyzed using analysis of covariance and spline regression models. Results: A significant linear trend was observed across the quartiles of BMI change for earlier age at diagnosis [Q1 52.3 (± 0.73), Q2 51.9 (± 0.70), Q3 49.6 (± 0.66), Q4 47.3 (± 0.67), p < 0.0001] after adjusting for potential confounders. In analyses that stratified by tumor subtype and menopausal status, significant linear trends continued to be observed for earlier age at diagnosis across quartiles of BMI for ER ± , PR ± , HER2 + , as well as pre- and postmenopausal status (p-values < 0.001). Conclusions: Women who gain excess weight during adulthood are not only at risk for breast cancer, but also may experience earlier onset of disease and reduced cancer-free years.